First public commit.

lenser/cpc/convert/mono.py

@@ -0,0 +1,47 @@
+"""CPC monochrome: Mode 2, 640x200, 2 colours (black + bright white) -- the
+highest-resolution CPC mode, tone carried by dithering.  ``--mono-base`` tints
+the second tone to a chosen hue."""
+from __future__ import annotations
+
+import numpy as np
+
+from ... import dither, palette as c64pal
+from ...convert.base import Conversion, perceptual_error
+from .. import palette as cpcpal
+from . import _common
+
+WIDTH, HEIGHT = 640, 200
+PIXEL_ASPECT = 0.5          # 640x200 on a 4:3 screen -> pixels half as wide
+NCOL = 2
+
+
+def convert(img_rgb, palette_name="cpc", dither_mode="floyd",
+            intensive=False, base_color=None):
+    plab = cpcpal.palette_lab()
+    # black + the lightest tone (bright white), or black + a tint
+    black = min(cpcpal.GREYS, key=lambda i: plab[i, 0])
+    if base_color in range(len(cpcpal.PALETTE)) and base_color != black:
+        ramp = sorted({black, int(base_color)}, key=lambda i: plab[i, 0])
+    else:
+        ramp = sorted(cpcpal.GREYS, key=lambda i: plab[i, 0])
+        ramp = [ramp[0], ramp[-1]]                  # black + brightest grey
+    # collapse to luminance so the two tones bracket the image's lightness
+    img_lab = c64pal.srgb_to_lab(img_rgb)
+    mono_lab = np.zeros_like(img_lab); mono_lab[..., 0] = img_lab[..., 0]
+    plab_mono = np.zeros_like(plab); plab_mono[:, 0] = plab[:, 0]
+
+    allowed = np.tile(np.array(ramp), (*mono_lab.shape[:2], 1))
+    idx = dither.quantize(mono_lab, allowed, plab_mono, dither_mode).astype(np.int64)
+    lut = {p: k for k, p in enumerate(ramp)}
+    pen = np.vectorize(lut.get)(idx).astype(np.uint8)
+    inks = [cpcpal.ink_byte(p) for p in ramp]
+
+    screen = _common.PACK[2](pen)
+    prgb = cpcpal.get_palette().astype(np.uint8)
+    return Conversion(
+        mode="mono", width=WIDTH, height=HEIGHT, pixel_aspect=PIXEL_ASPECT,
+        index_image=idx.astype(np.uint16), data=screen, data_addr=0xC000,
+        viewer="cpc", preview_rgb=prgb[idx.astype(np.uint16)],
+        error=perceptual_error(idx, mono_lab, plab_mono),
+        meta={"palette": "cpc", "dither": dither_mode, "cpc_mode": 2, "inks": inks},
+    )